1. Field of the Invention
The present invention generally relates to composing and playing multimedia presentations and, more particularly, to a flexible time stamp information carried in the stream descriptor of the multimedia presentation.
2. Background Description
Multimedia authoring systems exist that allow the user (i.e., the author) to insert multimedia objects, such as video, audio, still pictures, and graphics, into a multimedia presentation at a certain spatial position and with a certain temporal location. Such an authoring system is used typically to create presentations that are in an MPEG-4 (Motion Picture Experts Group, version 4) or SMIL (Synchronized Multimedia Integration Language) format.
In more advanced authoring systems, the temporal location of the multimedia objects need not be absolute in time, but can be defined relative to other multimedia objects. This means that, for example, a video clip can be authored to start at the same time that a specific audio clip starts. Another such example is that after completely playing a certain video clip, another video clip should be played, possibly with some delay. The essence of this is that multimedia objects have start and end times that are defined with respect to the start and end times of other multimedia objects, with possible temporal offsets (delays).
A further feature of advanced temporal authoring of multimedia objects is the possibility to have a range in duration of multimedia objects. For example, a certain video clip has a certain duration when played at the speed at which it was captured, say thirty frames per second. This now allows authors to define a range in the playback speed, for example between fifteen frames per second (slow motion by a factor of two) and sixty frames per second (fast play by a factor of two). This results in respectively a maximum and minimum total playback duration. In general, the advanced authoring systems allow authors to specify such ranges in multimedia object playback duration. Note, that it is still possible to dictate only one specific playback duration (which is directly related to the playback speed in the case of video, audio, or animation) by restricting the duration range to a zero width.
If we now combine the relative start and end times of multimedia objects in the authoring system with the possibility to also specify a duration range, we see that a complete authored multimedia presentation is a complex but flexible system of interconnected objects with variable durations. The advantage of having this flexibility in duration lies in the data transmission and playback of multimedia objects. By not having very strict multimedia start and end times, the system has some flexibility to adapt to data delivery problems, which may be due to network congestion or transmission errors. For the final delivery and playback the system (which may be the server or the client) will resolve the true multimedia object start and end times during transmission and playback adaptive to the environment.
In general, with these variable object durations, many actual values for start and end time are possible for all of the multimedia objects, especially when no delivery problems occur. In actual playback, absolute time stamps must be used. That means that for every multimedia object a playback duration is chosen which lies within the range of its possible durations. The problem of determining these factual durations at run time (i.e., playback) is addressed here. The method will be progressive in time; that is, it resolves the absolute time stamps as time advances, making it adaptive to the changing environment. Finally, it must be defined what information is to be sent to a client, that is sufficient to do the time stamp resolution.